495 PALAEONTOLOGY OF NEW-YORK. 



The shells of this genus are oval, ovoid or suborbicular, elongate or 

 rarely transverse ; valves unequally convex, with or without a median 

 fold or sinus; beak of the ventral valve often with a circular fora- 

 men, and incurved over the umbo of the dorsal valve. Area none ; valves 

 articulating by teeth and sockets ; surface smooth, or with fine concen- 

 tric lines of growth, and with very fine, indistinct or obsolete, radiating 

 striae. 



Since this shoeliftcr process, or septum, was originally described by Prof. Suess as characteristic 

 of his Genus Mekisia, and the species designated by him as the tyf)es of this genus (the M. scalprum, 

 M.herculea of Barkande ) "do possess this feature, the genus must be retained for the species 

 " with the shoclifler process." 



It would appear, therefore, that the Genus CAMAnii'M, proposed by me in the preceding Report, 

 possesses characters identical with Merista as originally described by Suess, but which have been 

 overlooked to some extent in consequence of the reference to.V. tumida as a typical form of the genus. 

 • • • • At the same time, as the .^7. tumida of Dalman, an English and Swedish species, in 

 common with numerous well-marked forms in our Silurian and Devonian strata, do not possess this 

 feature, we can no longer, with propriety, refer them to that genus. 



With this restriction, the JIerist.t: proper consist of smooth, ovoid, circular or transvcr.sc shells, 

 with usually a cons|>icuous sinus upon the ventral valve, and a corresponding wide, often undefined, 

 mesial fold or elevation upon the dorsal valve ; the hinge articulation being not very different from that 

 of Athvris, to which they are allied. 



The interior of the ventral valve, however, is strongly distinctive ; and the septum or shoeliftcr 

 process is not unfrequently shown in the cleavage of the beak of that valve, in solid specimens, where 

 the interior is inaccessible. 



The fonns which I have regarded as Merista are similar to those above ; but instead of this sep- 

 tum, or shoeliftcr process, they have a deeply marked triangular muscular area just below the rostral 

 cavity of the ventral valve, which is bordered on the anterior side by a callosity of the shell, and on 

 the two other sides by the strong dental lamella;. This feature is not conspicuous in ATiiyni.s : the 

 dental lamelUe in that genus are shorter and lews strong, and the form of the muscular impression is 

 different. The dorsal valve of tho.sc shells now under consideration has a longitudinal median septum ; 

 a feature which is obsolete, or partially obsolete, in the species of Athtris. In the Camarium, or 

 Mkrista proper, the exterior of the ventral valve sometimes shows what appear to be two diverging 

 septa, somewhat similar to those in the dorsal valve of Pentamekus, which are the margins of the 

 shoclifter process. 



The Mcristidaj begin their existence, so far as we know, in the rocks of the Clinton group ; and in 

 this and the Niagara group there are several species, while they are more numerous in the Lower 

 Ileldcrljcrg group : they occur likewise in the Upper Ilelderljerg rocks, and in the Hamilton group. 

 Merista proi)er, so far as we know, appears first in the Lower IIcMerbcrg period, while Atutris is 

 known in two sjwcics for the first time in the Hamilton group [also in Tjjpcr Ilelderberg grouj)]. 



Restricting, therefore, the signification of the Genus Mkrista to such forms as were originally 

 included by Prof. Suess under that name, it Iwcomcs necessary to designate those species of .s.imilar 

 form, but without the peculiar ai)pen<lage of the ventral valve, by another generic term ; and I would 

 therefore suggest the name Meristklla, projjoscd by me last year.J 



t Tneirtb Report on the State Cabinet, p. 78. 



